The present invention concerns a linear actuator.
By actuators there is the problem that the bushing at the end of its travel abuts on a surface in one of the tubes and thereby may be wedged. Hereby a further activation for moving in the opposite direction may be impeded or even made impossible. In order to avoid the bushing hitting end faces and being wedged, the actuators are provided with end stops which may be designed in various ways.
Thus it is known to provide actuators of the kind mentioned in the introduction with end stops which are furnished as sensors disposed at the outside of the tubes and which interact with magnets provided on the bushing. These end stops may be movable and disposed apart from the ends of the outer tube. As the sensors register the presence of the magnet, a signal is transmitted to the actuator motor which is deactivated. Hereby the continuous rotation of the spindle is stopped and engagement of the end surfaces is prevented. This actuator is, however, relatively expensive, and there will be a risk of failure if persons unintendedly bump on the sensors so that they are displaced and thereby give off signal at the wrong time.
Actuators are also known where attempts have been made to prevent wedging by providing the bushing with a rubber sleeve for abutting on end faces in the tubes. However, a rubber sleeve will not prevent the wedging, but only reduces the force to be overcome by activation for movement in the opposite direction. Thus there will still be need for a motor that may yield sufficient large force to overcome the wedging, and which hence becomes unnecessarily expensive. Furthermore, a wedging may still be so effective that movement in opposite direction is impossible.
Also, it is known with an actuator based on a slip coupling between the bushing and the housing. This actuator is based on a bushing which is built from a number of elements which via spring forces are to be held in predetermined positions. The construction is technically complicated and therefore expensive. Furthermore, the many interacting elements will imply risk of erratic performance.
A common feature of the prior art actuators is thus that they are relatively complicated and therefore expensive as well as there is a risk of faulty performance in the many interacting elements. For many years there has existed a need for actuators that are reliable and also inexpensive. For example, this is a need in various equipment for aiding the handicapped, such as wheel chairs with swivelling or pivotable seats and backs, and in beds with mutually pivotable sections to facilitate alighting and entering, and for making the bed comfortable for the user.
It is the object of the present invention to indicate an actuator of the kind mentioned in the introduction and where in a technically simple way there is provided security against wedging of spindle and bushing in extreme positions as slip may occur between the elements of the actuator.
This is achieved according to the present invention with an actuator which is peculiar as described in the characterising part of claim 1.
Preferably, the flange also has sloping lateral faces forming an angle between 15 and 60xc2x0 relative to a plane perpendicular to the centre axis of the spindle. Preferably, the pitch angle for the thread is less than the angle of the sloping lateral faces as measured relative to a plane perpendicular to the centre axis of the spindle.
In a preferred embodiment, the first tube is constituted by the outer tube and the second tube of the inner tube. However, it is within the possibilities of the skilled in the art to design the actuator within the scope of the invention so that the first tube is constituted by the inner tube and the second tube simultaneously is constituted by the outer tube.
In normal use, where the two tubes of the actuator are to be displaced mutually within their normal range, the difference in friction between spindle and bushing and between bushing and housing will cause the spindle to rotate inside the bushing and thereby causes the desired mutual displacing of the two tubes. In an extreme position, the end stop will prevent a further mutual displacement of the two tubes so that the friction between spindle and bushing momentarily rises very strongly and thereby becomes greater than the friction between the bushing and the housing. Hereafter the bushing may rotate freely together with the spindle as long as this is rotating in the same direction. Therefore, the bushing will not be pressed into a wedged engagement with an end surface in the tube.
In a preferred embodiment of the invention, though not delimited to this embodiment only, the end stop comprises stop means on the spindle and stop means on the bushing interacting by mutual impact, whereby the bushing overcomes the friction with the bushing housing and thereby can rotate together with the spindle by a continuous rotation of this.
When the rotation of the spindle is stopped and activated for rotation in the opposite direction, the difference in friction will again cause the spindle only to rotate and thereby the mutual displacing of the two tubes is established as the bushing does not rotate because of the frictional engagement with the housing.
The actuator is technically very simple as it only comprises the three interacting elements spindle, bushing and bushing housing for establishing the end stops. These elements are designed with geometrical shapes ensuring the correct functioning. These geometrical shapes may be calculated in simple ways from common algorithms for determining frictional connections and screw connections. From these algorithms moments may be determined so that for given loads of the actuator and for given dimensions of elements of the actuator there will always be a greater moment for the bushing (the slip coupling) than for the spindle.
According to an advantageous embodiment, the actuator is made in a particularly simple way in that the stop means of the spindle comprise a pin mounted in a radially extending boring and projecting outside the thread, and in that the stop means of the bushing comprises a pin mounted in an axially extending boring and projecting from the bushing in a radial position which makes possible engagement with the pin of the spindle. Alternatively, the stop means of the bushing may comprise an axially extending surface provided in a recess which is formed in an end face of the bushing and which has a radial extension allowing the pin of the spindle to protrude into the recess by rotation of the spindle. This recess may be formed as a boring in parallel with the centre axis for the spindle or as a helical recess which is ended with a radially extending side wall.
It is also possible that the stop means of the spindle comprise a pin mounted in an axially extending boring in a flange projecting from the spindle, and that the stop means of the bushing comprise a pin mounted in an axially extending boring and projecting from the bushing in a radial position making possible engagement with the pin of the spindle. This embodiment is particularly suitable for spindles with small diameter and consequently small amount of material in which a boring for the pin may be formed. In such a spindle there is risk of weakening the material by boring in the spindle.
In order to make the mounting so simple as possible, the bushing housing is preferably provided in two parts between which the recess is formed for accommodating the flange on the bushing, and that the two parts of the bushing housing are screwed together around the flange by means of a screw connection. The bushing housing is screwed fast at the end of the inner tube.